Besides the new episodes of the animated series, you can find new Sam & Max material in the "Adventurer" newsletter published by LucasArts.
Apparently Surfin' the Highway contains all the S&M material Steve Purcell has ever done, excluding some minor spot illustrations.
From the Comico issue: On The Road Road Trip Game Our Bewildering Universe From the (coveted) Epic issue: Bad Day on the Moon Disguise-O-Rama Roach Pin-up From the (highly coveted) Fishwrap issue: Monkeys Violating the Heavenly Temple Fizzball Night of the Gilded Heron Shark Getting Along in the Joint Others: Night of the Cringing Wildebeast Fair Wind to Java The Damned Don't Dance Beast from the Cereal Aisle Terror on the Tanbark Ponderings of the Ages Magic of Animation